Formation fluid flowing from a subterranean formation into a downhole tool is often a mixture of oil and water. Generally, the mixture is unstable and, therefore, the oil and the water separate over time if the mixture is static. Generally, to determine a viscosity of the oil in the formation fluid, a sample of the formation fluid is stored in a container until the oil separates from the water, or a chemical demulsifier may be added to the mixture to cause the oil and the water to separate. The oil may then be removed from the container, and a viscosity of the oil may be determined.